Musculoskeletal and connective tissue disorders are presenting devastating impacts on society. They are the most common causes of severe chronic, long-term pain and physical disabilities, affecting people of all ages and at a huge cost to society. One in every 3 women over age 50 will suffer from a fracture caused by osteoporosis. Every year, 23-34 million people worldwide are injured in road traffic accidents. Back pain is the 2nd leading cause of workplace sick leave. Up to 80% of people suffer from back pain during their lives. Osteoarthritis affects over 135 million people worldwide. It is the 4th most frequent cause of health problems in women and the 8th in men. There are about 30 million annual tendon and ligament injuries worldwide (Maffulli et al., Clin Sports Med 2003; 22:675-692). Chronic tendinopathy accounts for 30-50% of all sports-related injuries, and almost half of all occupational illnesses worldwide.
Diseases of the skin, musculoskeletal system and connective tissue are among the leading causes of hospitalization. In USA, about 2×105 tendon and ligament repairs are performed annually (Pennisi. Science 2002; 295: 1011). Anterior cruciate ligament (ACL) reconstruction is the sixth most commonly performed procedure in orthopaedics (Garrett et al., J Bone Joint Surg Am 2006; 88: 660-667) and the number one surgery in sports medicine.
Tissue repair after injury is very slow and inefficient. For example, tendons do not heal by a regenerative process but via formation of a fibrotic scar after injury with diminished mechanical strength, causing significant dysfunction and disability. Surgical reattachment of tendon and bone often fails and presents difficulty for tendon to bone healing due to the lack of regeneration of the enthesis. The failure rates for rotator cuff repair have been reported to range from 20% to 94%. Similarly, ACL does not heal spontaneously after injury. ACL reconstruction, which requires a tendon graft to be put inside a bone tunnel, has failure rate ranged 10%-25%, depending on the evaluation criteria used. Osteoporotic bone has diminished capacity to heal after fractures in aged subjects. Currently, there is no cure for osteoarthritis. Hypertrophic scars of the skin formed secondary to thermal or surgical injuries cause scar contracture and functional limitation of affected tissue, restriction of growth of children and cosmetic problem.
As a result, both acute and chronic tissue injuries are difficult to manage and can result in long-term functional disability and pain, which places a chronic burden on health care systems.
Tissue injuries are commonly managed either conservatively or surgically. For example, steroid injection; physical modalities such as low-intensity pulsed ultrasound, shockwave and physiotherapy are commonly-used conservative treatments for tendon/ligament injuries. The effects of these treatments are sometimes palliative and the treatment time is usually long. If the conservative treatments fail, surgery is required for repairing the injured tissue. When the injury is severe enough, autografts, allografts, xenografts and prosthetic devices may be needed for repair or replacement of damaged tissues but have shown limited success. These methods have some inevitable disadvantages such as donor site morbidity, risk of disease transmission and tissue rejection, and limited long-term function/durability. Therefore, tissue engineering is receiving increasing attention as a potential strategy for the treatment of tissue injuries.
Tissue engineering was once categorized as a discipline under biomaterials, but has grown in scope and importance in recent years and now it is a special field in its own. It is defined as the use of a combination of cells, biomaterial and suitable biochemical and/or physio-chemical factors for repair or replacement of biological tissues. The use of tissue engineering approach for the development of functional replacement tissue for clinical use has the potential to promote tissue repair, improve the quality of healing for full restoration of tissue function and reduce the chance of tissue re-injuries.
Cells and scaffold are the two most essential components of tissue engineering. Among different possible cell sources for tissue engineering, mesenchymal stem cells (MSCs)-derived from the bone marrow (BMSCs) were commonly used (Chong et al., J Bone Joint Surg Am 2007; 89: 74-81; Hankemeier et al., Arch Orthop Trauma Surg 2007; 127(9): 815-821) because these cells maintain some degree of self-renewal potential and have the capacity to differentiate into cells of multiple mesenchymal lineages. The synthetic and proliferative abilities of these cells are also robust (Liu et al., Biomaterials 2008; 29: 1443-1453; Ge et al., Cell Transplant 2005; 14: 573-583). Despite these encouraging findings, there is a chance that the MSCs might not differentiate towards the target tissue type or induce tumor formation. For instance, transplantation of BMSCs into the rabbit tendon defect was reported to form ectopic bone and expressed alkaline phosphatase activity in constructs (Awad et al., J Orthop Res 2003; 21: 420-431; Harris et al., J Orthop Res 2004; 22: 998-1003). Tumor induction by undifferentiated BMSCs was reported in some specific circumstances (Tasso et al., Carcinogenesis 2009; 30(1): 150-157). The in vitro differentiation of stem cells towards tissue-specific lineage before transplantation might be a good strategy to promote tissue healing while minimizing the chance of erroneous cell differentiation and tumor induction. However, a method of controlling the differentiation of MSCs to target progenitor cells remains a great challenge which hinders their application.
Although stem cells isolated from different tissues share many important stem cell characteristics, certain properties of stem cells are affected by their origins (Sakaguchi et al., Arthritis Rheum 2005; 52: 2521-2529). The selection of appropriate stem cell source is important for successful tissue engineering. Recently, stem cells have been isolated in tendon (Bi et al., Nat Med 2007; 13(10) 1219-1227). The inventors were the first group to report the isolation and characterization of tendon-derived stem cells (TDSCs) from rat (Rui et al., Tissue Eng Part A 2010; 16(5): 1549-1558). The tendon stem cells present new opportunity for repairing damaged tissues and bio-artificial tissue engineering.
As mentioned, in vitro differentiation of MSCs into tissue-specific progenitors prior to transplantation is a possible approach to circumvent the problems of erroneous cell differentiation and tumor formation while promoting tissue regeneration. Different factors have been reported to have effects on cell differentiation including growth factors, mechanical stimulation, composition and topographical cues from biomaterials, co-culture with tissue-specific cell types and gene modification. Hence they might be suitable for in vitro differentiation of MSCs into tissue-specific progenitors.
The composition and properties of biomaterials used as scaffold for tissue engineering can significantly affect the regeneration of neo-tissues. There are different categories of scaffolding materials for tissue engineering: polyesters, polysaccharides and collagen derivatives and calcium phosphate derivatives.
Synthetic scaffolds such as polyglycolic acid (PGA), polylactic acid (PLA) and their copolymer polylactic-co-glycolic acid (PLGA) have been used for bone and tendon tissue engineering. They are attractive because their degradation products, glycolic acid and lactic acid, are natural metabolites and they have good mechanical properties as well as outstanding processability. As they are manufactured from chemical compounds, they permit better control of chemical and physical properties and hence quality. However, biocompatibility of these synthetic scaffolds is very poor as they do not support a high level of cell adhesion (Zhu et al., J Biomed Mater Res 2002; 62: 532-539) and the natural metabolites are acidic in high concentrations which may result in local inflammatory reaction.
On the other hand, biological scaffolds such as relatively pure natural collagen derivatives and the complex decellularized extracellular matrix materials with multiple natural macro-molecules such as small intestinal submucosa (SIS) (Dejardin et al., Am J Sports Med 2001; 29: 175-184) and silk (Altman et al., Biomaterials 2002; 23(20): 4131-4141), are highly biocompatible and also exhibit superior bio-functionality. Limitations of these biological scaffolds are low mechanical properties (Gentleman et al., Bioamterials 2003; 24: 3805-3813). Their processabilities are also limited (Gentleman et al., Bioamterials 2003; 24: 3805-3813). High batch-to-batch variation also makes a reliable production of these scaffolds difficult (Koski et al., Orthop Clin Nort Am 2000; 31: 437-452). They may cause inflammatory response, implant rejection (Koski et al., Orthop Clin Nort Am 2000; 31: 437-452) and have risk of disease transmission (Chen et al., Stem Cells 2009; 27(6): 1276-1287).
Calcium phosphate derivatives such as the hydroxapetite and tri-calcium phosphate (TCP) are commonly used as scaffolding materials for bone regeneration. These materials usually take long time for degradation.
Accordingly, a need exists for finding the appropriate cell types and scaffold materials for the promotion of tissue repair or engineering of bio-artificial tissue for replacement of damaged tissue.